Certain inspection equipment performs raster scans on semiconductor wafers. An example of such inspection equipment is a confocal microscope-a kind of microscope which focus is fixed during operation. A confocal microscope can obtain enhanced optical quality if the optical system is fixed and the specimen under inspection is made to move with respect to the optical system. Thus, during operation, the movement of the specimen allows a designated area of the specimen to be scanned by the fixed optical focus. FIG. 1 shows part of an implementation of such a confocal microscope 100 with a samiconductor wafer specimen 104.
As shown in FIG. 1, light is transmitted through lens 102 and focussed on to a point 103 inside a semiconductor wafer 104. Semiconductor wafer 104 is shown in FIG. 1 as a cross section. From focal point 103, the transmitted light diverges and leaves the semiconductor wafer 104 before being transmitted by lens 105 to the signal processing system (not shown) of confocal microscope 100.
Confocal microscope 100 operates in transmission, i.e. the light used is chosen from a range of wavelengths at which semiconductor wafer 104 is transparent. For a silicon wafer, one such wavelength is 1.3 microns.
In one inspection application, one narrow section of semiconductor wafer 104 is scanned in a plane perpendicular to the surface of the wafer. To achieve this raster scan, the stage is moved through a number of fine steps in the vertical direction, so that focal point 103 of confocal microscope 100 moves through various depths in semiconductor wafer 104. At each step, semiconductor wafer 104 is moved back and forth along an axis parallel to the semiconductor wafer surface. The optical signal transmitted through the semiconductor wafer is provided to form a raster scan image.
For this technique to work, it is necessary that the wafer is scanned back and forth along a plane parallel to the semiconductor surface in a rapid, smooth and reproducible manner.